The Fraunhofer diffraction particle sizing technique has become well accepted for characterizing both solid and liquid particles. This method is an ensemble technique which overages over the line of sight of the laser beam and this attribute is often considered a disadvantage in that the axial, that is, along the laser beam, spatial resolution is very poor.
The method disclosed here is based on analyzing the signature of light scattered in near-forward directions (i.e. in directions close to the progagation direction of the illumination beam) by ensembles of particles to determine various properties of the particle population. Since scattering angles are by convention measured from the forward direction, the near-forward scattering directions are small angles. Also under certain common conditions, specifically particles large compared to the wavelength with refractive indices significantly different than the surrounding medium, the near-forward or small angle scattering properties are readily predicted or approximated by Fraunhofer diffraction theory. The relevant methods of interest are also referred to as "Fraunhofer diffraction technique".
The art of particle sizing and more particularly, the evolution of means for determining the concentration and size distribution of particles in a liquid or a gas using near-forward scattering patterns, which means is also capable of remotely measuring these properties with axial spatial resolution is a primary focus of the present invention.
Many commercial processes would benefit from on-line monitoring of liquid and gaseous suspensions. For example, the ability to characterize the size distribution of dispersed particles and droplets is of crucial importance in a number of practical systems. Some important applications include: liquid fuel droplets sprayed into air in combustion systems such as boilers and gas turbine combustors; solid particles dispersed in liquids as in coal-oil slurries; solid particles dispersed in combustion exhausts with respect to the health aspects of particulate pollutant emissions; and others. In many of these applications, optical (as opposed to batch) sampling techniques for particle sizing are advantageous and sometimes necessary. (The term particle will refer herein to both solid particulate matter and fluid droplets of diameters approximately 0.01 .mu.m to 1 mm.)
A problem which is often encountered in measuring techniques is to determine the size distribution of physical entities, such as particles in a liquid or gas, gas bubbles in a liquid, or liquid droplets in liquids or gases. All of the various measurable entities will be herein referred to as "particles" and all references to "particle measurement" is intended to include the measurement of gas bubbles and droplets as well. This task is addressed and, to some extent alleviated by the systems described in a number of U.S. Patents, the disclosures of which are specifically incorporated herein by this reference thereto. The U.S. Patents referred to are: U.S. Pat. No. 3,469,921, Taylor; U.S. Pat. No. 3,636,367, Girard; U.S. Pat. No. 4,037,964, Wertheimer et al; U.S. Pat. No. 4,338,030, Loos; U.S. Pat. No. 4,251,733, Hirleman (I); U.S. Pat. No. 4,188,121, Hirleman (II); U.S. Pat. No. 3,835,315, Gravitt; U.S. Pat. No. 3,689,772, George et al; U.S. Pat. No. 3,988,612, Palmer; U.S. Pat. No. 4,360,799, Leighty; U.S. Pat. No. 4,740,677, Carreras et al; and U.S. Pat. No. 3,873,206, Wilcock.
Advanced optical systems for determining the particle parameter of size often use laser illumination of single particles and analysis of the scattered light characteristics to obtain information on the size and other physical parameters of a given particle. The sizes of many particles are measured and summed to determine the particle concentration and the overall particulate size distribution. The use of lasers is advantageous due to the greater light intensity available as compared to conventional light sources, thereby allowing measurement of smaller particles and enhancing the ability for in-situ or non-interfering measurements. Arrangements using white light scattered in only one solid angle require an extremely well defined and compact sampling volume through which a representative sample of the particulate flow must be passed.
In the system disclosed by Gravitt, supra, laser or other light is focused to intensely illuminate a small region in space. This region, called the sensitive volume or particle sampling zone, is located in the field of light collecting apparatus which discriminates between the light scattered at two small angles and the light traveling in the light beam propagation direction. Detector means are used simultaneously to detect and record signals representing the intensities of the scattered light detected at the different angles. A measure of one of the parameters, i.e. the particle size, of a particle passing through the sampling zone is determined by measuring the ratio of the signals representing the intensities of the scattered light detected at two angles. This measurement is, however, non-unique or ambiguous since particles of different sizes may pass through the sampling zone and since many particle sizes can generate the same ratio signal.
One problem with a laser system is the Gaussian intensity distribution in the beam, since single angle systems can not differentiate between a small particle passing through the high-intensity center of the beam and a larger particle passing through an off-center point of lower intensity. This problem can be eliminated by utilizing the ratio of light intensities scattered in two directions thereby cancelling the incident intensity effect as suggested by Gravitt.
Hirleman I discloses a technique for measuring particle size and velocity using two beams of electromagnetic radiation with symmetrical radial intensity distributions directed through space. A particle sampling volume is defined by those portions of the two beams within the field of view of one or more radiation sensitive detectors. The detectors respond to scattered radiation or fluorescence from particles passing through the beams in the sampling volume. The detector output for a single particle indicates two signal pulses corresponding to those times when the particle was in one of the beams. The speed of the particle in the plane perpendicular to the beams is determined from the transit time or width of the signal pulses, and the angle of the particle traverse in that plane determined from the time-of-flight between the signal pulses.
Hirleman II discloses an improved multiple ratio single particle counter in which intensities of scattered radiation are measured at more than two angles and the ratios of these intensities are derived. The derived ratios are then compared with calibration curves to determine an unambiguous measure of the particle parameter.
The family of methods which use the aggregate scattering properties of an ensemble or population of particles are herein termed "ensemble scattering" techniques.
The simplest example of an ensemble scattering method using near-forward small-angle scattering comprises a light transmitter creating a beam of light, a sample through which the beam of light is directed, and a plurality of discrete detectors disposed in a preselected angular relationship to the central axis of the light beam and to each other. The photodetectors are placed at a multiplicity of angles and collect light scattered in the various directions. Equivalently, a single detector might be translated (as in a goniometer) through various angles and the scattered light readings recorded. To obtain a particle-position-independent response from a conventional system as described above requires that the detectors be disposed very far away from the particles, where "far away" means would require distances many times greater than the extent of the illuminated portion of the particle field which is often impractical. However, the placement of a lens on axis in the scattered light field will effectively convert an angular scattered light distribution in the far-field to a radial distribution at the back focal plane of the receiving lens. This was done by Chin et al in 1955. (See: Journal of Physical Chemistry, vol. 59, 1955, p. 841. ) It turns out that the E-field distribution of the scattering signature at the back focal plane of the lens is the Fourier transform of the E-field distribution, one focal-length in front of the lens. For this reason, the back focal plane is called the "Fourier transform plane" or the "transform plane". It also occurs that the scattering at the transform plane is essentially independent of the position of the scattering particle(s). For that reason detection of the scattering is often performed at the transform plane, and hence this plane is also often called the "detection plane". The particle measurement art then includes a scheme of segregating and detecting the scattered light irradiance at a multiplicity of positions in the detection plane. Chin et al, supra, traversed a small photodetector behind a small aperture across the detection plane to register the scattering signature.
In ensemble scattering particle sizing it is necessary to measure light scattering at a multiplicity of angles to infer particle size distribution. Generally in the optical system, the transform lens converts the far-field angular diffraction pattern into a spatial distribution of scattered light at the transform or detection plane. In the prior art, there were a number of concepts developed for sampling the scattering or diffraction pattern. In the earliest work, researchers translated a single detector with a pinhole aperture across the diffraction pattern to obtain measurements at roughly even increments of the scattering angle. A major shortcoming of this technique arose from the fact that the intensity in the diffraction pattern drops off rapidly from the near-forward (near on-axis) angles to larger off-axis angles. This results in a signal dynamic range which is often too large for a single detector in practical environments where noise is a problem. Similar difficulties are encountered when a solid state detector array is used having equal area detector elements.
A very general method to compress the dynamic range required of detectors is to utilize a detection strategy whereby the detector aperture(s) increase in area as the distance from the optical axis (diffraction pattern center) is increased. This approach provides the largest area in those regions of the diffraction pattern where the intensity is the lowest.
The ring detector comprises an array of concentric annular detectors on a single silicon wafer with areas which increase with radial distance from the detector center. This detector which had a series of wedge-shaped detector elements on the other half, was manufactured by Recognition Systems, Inc. (RSI).
Since the Fraunhofer diffraction pattern possesses circular symmetry, the rings and wedges sample the diffracted energy in polar coordinate form. That is, the rings sample the distance of the diffraction pattern portions from the axis, while the wedges sample the direction at which portions of the pattern are disposed. A suitable wedge-ring detector, having 32 rings and 32 wedges, is disclosed by George et al in U.S. Pat. No. 3,689,772.
Palmer, cited above, discloses a photodetector array system in which the array is comprised of a matrix of photodiode detectors, and may, for instance, be a 32 by 32 element device such as the Reticon model R32X32A.
Loos describes an arrangement for measuring the size distribution of particles suspended in a gas or in a liquid. In Loos, a spatial filter is placed in the exit plane of a dispersive element so that its transmittance is a function of position on the filter. Light transmitted by the filter is measured by a photodetector. The photodetector output is measured as different spatial filters are switched in place.
Wilcock and Wertheimer et al, supra, discuss a Fourier transform plane spatial filter in which a mask lies in the transform plane of the lens while in Taylor, supra, the size distribution of an aggregation is determined by the amount of light in a ring in the Fourier plane. Girard describes a Fourier transform optical analyzer which uses a mask shifted step by step relative to an optical object support.
While the above-cited references are instructive, the task of measuring particle size distribution in liquids and in gas, particularly in a manner which permits axial spatial resolutions to be obtained, remains an ongoing need. These methods are all line-of-sight methods in which all particles in the laser beam scatter light into the detector plane.
Some efforts have been made to overcome this deficiency by taking multiple independent measurements by passing the laser beam through various sections of the particle field. Abel inversions (for symmetric aerosols) or tomographic reconstruction methods are then used to obtain spatially resolved data. In addition to being a convoluted procedure, these later techniques still leave uncertainty to the credibility of the data obtained. Thus, a need still exists for the provision of a relatively quick and highly credible procedure for obtaining axial spatial resolution in Fraunhofer diffraction particle size measurements. It is toward the resolution of this need that the present invention is directed.